Measuring pKa

The book is organized into eight chapters. Chapter 1 describes the physicochemical needs of pharmaceutical research and development. Chapter 2 defines the flux model, based on Fick s laws of diffusion, in terms of solubility, permeability, and charge state (pH), and lays the foundation for the rest of the book. Chapter 3 covers the topic of ionization constants—how to measure pKa values accurately and quickly, and which methods to use. Bjerrum analysis is revealed as the secret weapon behind the most effective approaches. Chapter 4 discusses experimental... 

Therefore, a continued interest exists in the role of pKa in absorption, which often is related to its effect on lipophilicity and solubility. New methods to measure pKa values are being explored [36], e.g., using electrophoresis [37], and an instrument for high-throughput pKa measurement has recently been announced [38]. 

To measure pKa values it is necessary to expose the compound to an environment of changing pH, and to monitor a particular property that changes as a function of the ionization state of the molecule. Traditional pKa methods have included solubility and conductivity methods [31]. These are slow, and require both large quantities of material and skilled operators. Some more rapid methods... 

Box, K. J., Comer, J., Peake, C., A new instrument for measuring pKa values and partition coefficients (log P), Poster at Brighton Crop Protection Conference, 1993. 

Many of the inorganic oxoacids are strong (i.e. have negative p/Ca values) in aqueous solution. But, as we have seen, use of a solvent with a lower proton affinity than water (for example pure ethanoic (acetic) acid makes it possible to differentiate between the strengths of these acids and measure pKa values. The order of strength of some typical oxoacids is then found to be (for H X -> H., X- + H+) ... 

Often the measure of hydrophobicity desired is an apparent log P of an ionizable solute at a pH where it is partly ionized and thus more hydrophilic. This is most often termed a distribution coefficient and reported as Tog D. These can be calculated from a measured pKa and the neutral log P, if it assumed that the octanol phase contains a negligible portion of the ionic species. This is a good assumption if the pKa of the solute is no more than 3.0 log units on the ionized side of the pH of measurement. 

S NMR spectroscopy could be a simple method to measure pKa values of sulphonic acids in aqueous solutions. Indeed, it was shown that the 33S chemical shifts of 3- and 4-substituted benzenesulphonates are linearly related to pKa by the following equations,59 61 derived respectively at 293 and 312 K ... 

Scheme 2.14 Hydrocarbons can be deprotonated and have measurable pKa values.

PCA200 entry-level measures pKa, log P, and intrinsic solubility... 

The pKa of a compound may be estimated using ACDpKa software, which also contains a large database of measured pKa data. The use of the computer program SPARC (scalable processor architecture) for estimating the pKa of pharmaceuticals has been described by Hilal etal. (1996). 

If the pH Of the solution and the concentrations of HB and B are measured, pKa can be readily calculated. Spectrophotometry is employed in determining the ratio (chb/cb ). Since the conjugate acid and base forms of a substance will be in equilibrium, the relative concentration of each form will depend on the pH of the solution. For moderately weak acids or bases, the pH of the solution may be adjusted so as to make one of the forms predominant. For example, the spectrum of an acid HB in alkaline solution will correspond to the absorption of the anion ( >b ) and in acid solution, to that of the undissociated acid (Dhb). At intermediate pH values, however, both forms will be present and the observed absorption (D) will be a linear combination of those of the two forms (provided Beer s law holds). Since the optical densities (jDb- and Dhb) of the two forms (for a fixed concentration of the solute) are known, the relative concentrations of the two forms at any intermediate pH may be calculated. A similar spectrophotometric procedure may be applied to bases. The acidity function, H0, which is a measure of the proton-donating tendency of a medium towards a neutral base, has also been determined by spectrophotometry85. 

From the measured pKa values of n-PrSH in water and (CH3)2SO (8), we have... 

X0 groups are acid strengthening as shown by the measured pKa values of 1.83 and 2.58, respectively. 

A difficulty in the use of glass pH electrodes, even in aqueous solutions, but especially in partly aqueous solutions, is that they are relatively slow to respond to changes in hydronium ion activity. Equilibration times of at least 1 min are generally needed for each data point in a titration that might have a total of more than 20 points. This has two consequences (a) potentiometric titrations are relatively time consuming and thus (b) workers can become too impatient to wait for the electrode response to fully stabilize, leading to further measurement errors. Automated measurement, such as with Sirius autotitration equipment, is able to measure about 25 pKa values per day. Faster alternatives to the pH electrode are needed to rapidly measure pKa values. 

NB Thermodynamic value refined for activity coefficients, and [OH-]. Results validated through measured pKa values for benzoic and 5,5-dietfiylbarbituric acids in agreement with best literature values. 

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